With all due respect to the good posters here, I'd like to offer an angle on this I don't think has been entirely addressed. Most of the responses here have been about Park->car, but very little has included info about Resort->Car->Park. That's a HUGE element in the equation in my book.
If, for example, you're staying in Saratoga Springs, you are rolling the dice with the buses. For us, the variability of the buses has proven to be the only real reason we would consider NOT staying there. As we normally get to the parks pretty early, it hasn't been too much of an issue, but for our midday break when we're tired, waiting for the bus is *not* fun
while your standing at the bus terminal at the park, especially when you see two, five, or (seemingly) 100 buses for
every OTHER resort go by (okay, okay, I exaggerate, but you get the point).
So, the question of driving/busing must consider where you're staying as much as anything else, so there really can't be a universal answer. We did, in fact, drive a couple of times from SSR to Epcot and MK, and realized its not entirely a panacea. There's the walk from the lot to the gate, and if you're a distance out, you may need to wait for a tram (either way). If you're staying at a resort with >1 transport option, eg monorail, bus, boat, the need for a car diminishes greatly IMHO.
And, frankly, there are just times when its nice to just flop on the bus and not worry about driving, esp. when you're the one doing the driving
